When there is an abnormality in a magnetic medium, an error may be generated in write data or read data stored on a hard magnetic disk (referred to as a “magnetic disk” or simply “disk”, hereinafter) used in a computer system. Therefore, a magnetic medium is certified by writing predetermined test data, for example, FFh data, on predetermined tracks of the magnetic disk and reading it by a certifier.
Incidentally, h of FFh indicates hexadecimal notation and FFh means data including bits all of which are “1”. A test data having a specified frequency signal of this kind is known as a “test burst signal”.
A bit error to be detected by the certifier includes a mixing error (including a bit having a level lower than a predetermined threshold value), spike error, positional modulation error and magnetic modulation error, etc. As another form of error, there is an extra error which is a bit detected after a data on the disk has been erased.
In a disk drive, a coil type magnetic head (inductive head) is used to write data and an MR head is used to read data. The write head and the read head are integrated as a composite head. The recording densities of the disk drives continue to improve with time.
Testing of disks for defects is performed by using a concentric circle test method or spiral test method. In the concentric circle test method, the disk defect is detected by searching respective tracks by radially moving a magnetic head mounted on a head carriage stepwise correspondingly to the respective tracks while rotating the disk. In the spiral test method, the disk defect is detected by scanning tracks of a rotating disk by continuously moving a magnetic head spirally with respect to the rotating disk.
The efficiency of the concentric circle test is low since it takes a long time in order to test the entirety of the tracks. However, the number of test tracks has increased recently due to improvements in the disk arts. Practically, the number of defective tracks is in a rage from 100 to 200, at most, which corresponds to 1/10 to 1/100 of the all tracks. Therefore, the spiral test or a thinned concentric circle test in which test tracks are partially thinned is usually used. Incidentally, the spiral test can be performed by thinning pitch.
Such techniques are described in JP-A-10-275434, JP-A-2000-57501 and JP-A-2000-57502.
In the concentric circle test method, writing of a test burst signal in one track is started by an index signal (or a sector signal) indicating a reference position of the disk and is ended by an index signal (or a sector signal) after one rotation of the disk. Therefore, a connecting region appears between the write start position and the write end position of the test burst signal.
The connecting region occurs for two reasons. One of the reasons is that the peripheral speed of a rotating disk is slightly different for every rotation and the write start position and the write end position do not match completely. The other reason is that there is a predetermined positional deviation (3·m to 5·m) between the read head and the write head.
Therefore, even if the magnetic head is controlled such that the write start position and the write end position are completely coincident, there is a deviation of the read signal correspondingly to the read head. It is practically difficult to perform control such that the positional deviation between the read head and the write head is corrected while the peripheral speed of the disk is varied slightly. Therefore, the connecting region C in FIG. 4(a) appears in the read signal for one track. Incidentally, FIG. 4 shows the generation of timing for a conventional test inhibit gate signal.
When the connecting region C is tested in the disk certifier, the connecting region C is detected as an error. Therefore, a test inhibit gate signal 14 having a window width covering the connecting region C of the read signal shown in FIG. 4(a) is generated to indicate that a portion (the connecting region C) of a track of the magnetic disk is a non-test region. Incidentally, a low level “L” of the inhibit gate signal 14 or an inhibit gate signal 18 to be described later is significant and this period is used as a test inhibit period. During this period, a read operation of a read circuit portion of the test signal write/read circuit is inhibited in order to invalidate the read signal itself.
In FIG. 4(a), reference numeral 11 depicts an index signal, 12 a write gate signal, 13 a read signal of a test burst signal written in one track and 14 an inhibit gate signal. Reference numeral 15 depicts a read gate signal which may be generated by inverting the write gate signal 12.
Incidentally, it is possible that writing of the test burst signal for one track is started not by the index signal 11 but instead by a sector signal as a reference and is ended by the same sector signal after the one track. An embodiment of this invention to be described later uses this system.
In the defect test of a disk, it is usual that the width of the connecting region C which is determined as the test inhibit period is set to a maximum with respect to a track to be tested under consideration of the maximum variation of the peripheral speed of disk and a positional deviation between the read head and the write head. Therefore, the window of the inhibit gate signal 14 is set with respect to the connecting region C having a maximum width corresponding to the rotation number and the peripheral speed of the disk by using the write end point as a reference.
FIG. 4(b) shows a positional relation of the MR head. Assuming that the positional deviation between the read head and the write head is A[m], the window width W [sec] of the inhibit gate signal 14 is selected such that the relation A[m]/peripheral speed [m/sec]<W [sec] is established.
Assuming that the rotation number of the disk is within a range from 2000 rpm to 3000 rpm (corresponding to the peripheral speed in a range from 4.1 m/sec to 6.3 m/sec of a disk whose radius is about 20 mm) and the positional deviation A between the read head and the write head is within a range from 3·m to 5·m, the window width W becomes about 1.5·m. This is the usual and currently set window width W.
When the recording density of one track increases, an amount of information recorded in the width of 1.5·sec of the non-test region is increased correspondingly. Therefore, in order to guarantee disk quality, it is requested to reduce the width of the non-teat region to as small a value as possible. However, when the width of the non-test region is reduced, a defect detection error rate increases and the number of disk which are re-tested increases, therefore resulting in decreases in testing efficiency.